There is an increasing need for reduced sulfur levels in petroleum and chemical streams due to increasingly stricter environmental requirements. This is especially so for middle distillate fuels such as diesel fuel, jet fuel, kerosene, heavy furnace oils and the like, whose combustion products are released into the environment. In hydrocarbon streams derived from petroleum refining and chemical processing, sulfur is present in the form of organosulfur compounds and is typically removed by a process known as hydrodesulfurization. In this process, the stream is contacted with hydrogen, in the presence of a suitable catalyst, to convert the sulfur in the organosulfur compounds to H2S. The H2S is then separated from the sulfur-reduced stream. Simple, sulfur-bearing organic compounds, such as aliphatic, naphthenic, and aromatic mercaptans, sulfides, di- and polysulfides and the like, are relatively easy to hydrodesulfurize and the sulfur in these types of compounds, being relatively easy to remove, is therefore referred to as “labile sulfur”. It is much more difficult to remove sulfur from more refractory sulfur compounds, such as derivatives of dibenzothiophene, especially those mono- and di-substituted and condensed ring dibenzothiophenes that exhibit steric hindrance, and sulfur in these compounds is referred to as refractory sulfur. These highly refractory sulfur heterocycles are present in the higher boiling [e.g., about 500 to about 800° F. (260-427° C.)] fractions of middle distillate streams and resist hydrodesulfurization, as a consequence of the steric inhibition precluding the requisite catalyst-substrate interaction.
Removing sulfur from these refractory sulfur heterocycles can be achieved under relatively severe hydrodesulfurization process conditions, but this requires high temperatures and pressures, is expensive, and can result in product loss due to cracking and degradation. As a consequence, processes have been developed to remove these refractory sulfur heterocycles from streams that have previously been substantially desulfurized, and typically by conventional hydrodesulfurization. For example, U.S. Pat. Nos. 6,193,877 and 6,245,221 relate to hydrodesulfurizing such sulfur-reduced streams, which still contain refractory sulfur heterocycles, in the presence of an H2S sorbent and a catalyst comprising noble metal or nickel on a particulate support. U.S. Pat. No. 6,251,262 discloses the use of three separate stages and catalysts to remove them, to produce product having about 0.005 wt. % (≦50 wppm) or less sulfur. This is still too high for many specifications. U.S. Pat. No. 6,210,564 discloses the use of sodium metal to remove sulfur from petroleum-derived feeds, but this requires special handling and forms sludge, which must be separated from the treated oil. The literature discloses the use of free and supported sodium for removing thiophene from naphtha, but doesn't address or suggest that this would be effective for the higher boiling, refractory sulfur heterocycles found in higher boiling streams. Examples are found in (i) B. M. Vanderbilt, “Desulfurization and Refining of Naphthas by Metallic Sodium,” Ind. & Eng. Chem., v. 49, n. 4, April, 1957 and (ii) Gerlock, et. al. in “Reaction of Thiophene with Sodium on Alumina. A Method for Desulfurization of Volatile Fuels,” Ind. Eng. Chem. Fundam., v. 17, n. 1, 1978. There is a need for a process that can remove sulfur from these refractory sulfur heterocycles and particularly from middle distillate streams, without resorting to expensive catalysts, difficult processes employing multiple catalytic stages and catalysts, and severe hydrodesulfurization conditions.